This invention relates to the manufacture of a device for clamping a generally cylindrical object, using plastic thermoforming processes, and more specifically to the low cost manufacture of clamping devices that conform closely to human limbs to provide support for various forms of traction therepy. Such clamping devices are known, and one is described in U.S. Pat. No. 3,380,447 (Martin), in which a cylindrical clamping device is made of metal, and is used for supporting a human in inverted posture by clamping the ankles and suspending the clamps from a horizontal bar by means of hooks.
The variety of sizes and forms of human limbs, through the range from children to large adults, makes it desireable to make traction clamps through a process employing simple, low cost tooling. Plastic thermoforming is well known for this advantage, and hence is found in use for making a variety of splints and body casts that are fitted to individual patients, using inexpensive molds. Although human limbs are generally cylindrical, clamping devices for traction should be shaped as closely as possible to the contour of the limb so that padding within the clamp provides the most uniform unit pressure possible to the skin of the patient.
The natural flexibility of plastics normally used for thermoforming helps the clamp to conform anatomically to the limb. However, thermoforming is limited to shapes that have little or no negative draft on the mold; meaning that a cylindrical object, such as a clamp cannot be directly formed by that process. Therefore a secondary forming operation is necessary to form a complete cylinder.
Local heating and bending is well known in the plastics industry, and is commonly referred to as "hot-wire" bending. In this process a flat sheet of plastic is placed over a hot electrical resistance wire until the localized radiant heating softens the plastic sheet along the intended bend line. A bend is made, and the plastic is cooled in the bent configuration.
Local stretch-thinning is also well known, and is shown by my U.S. Pat. No. 3,760,178 which teaches the use of thermoforming to intentionally thin a plastic web. Stretch-thinning of plastic for the purpose of providing a flexural hinge line is shown in my co-pending application Ser. No. 06/142,673 entitled Thermoformed Plastic Hinge.